This invention relates to a fault tolerant computer system which comprises three or more central processor modules and from which a processor module is isolated when a fault is present therein.
Various fault tolerant computer systems are known. In one of the fault tolerant computer systems that is disclosed in Japanese patent prepublication (A) No. 283,954 of 1986, each central processor module tests itself whether or not a fault is present therein.
In the manner which will later be described in greater detail, such a fault is detected in general by dynamic redundancy with one of the processor modules isolated in which the fault is detected. The dynamic redundancy is classified into hybrid redundancy, self purging redundancy, and shift out redundancy. In accordance with the hybrid redundancy and the self purging redundancy, a majority decision circuit is used in common to the central processor modules. According to the shift out redundancy, a multiplexer is used in common to the central processor modules.
In such a conventional fault tolerant computer system, an additional fault is unavoidable either in the majority decision circuit or in the multiplexer in addition to the fault in the central processor module. The additional fault gives rise to system down of the fault tolerant computer system and gives a reduced reliability to the fault tolerant computer system.
In accordance with the hybrid redundancy, the fault tolerant computer system includes a noncoincidence detector and a switching circuit. According to the shift out redundancy, the fault tolerant computer system includes a comparator and a detecting circuit. Each of the noncoincidence detector, the switching circuit, the comparator, and the detecting circuit serves as an external circuit common to the central processor modules of the fault tolerant computer system. On giving a greater scale to the fault tolerant computer system, the external circuit must accordingly be modified. Use of such an external circuit is therefore objectionable in order to give a flexibility to the fault tolerant computer system. This is a problem common to the hybrid redundancy, the self purging redundancy, and the shift out redundancy.